The technology is well developed for fabricating optical fibers, particularly optical fibers made of silica. The dominant process deposits a soot or other material inside a silica tube by CVD (chemical vapor deposition) or flame hydrolysis. The CVD process is disclosed by MacChesney et al. in "A New Technique for the Preparation of Low-Loss and Graded-Index Optical Fibers," Proceedings of the IEEE, volume 62, 1974, pp. 1280-1281 and in U.S. Pat. Nos. 4,217,027 and 4,334,903. The soot is then fired so as to form a thin uniform glassy layer of silica on the inside of the tube. This structure is referred to as the preform tube. The innermost silica, for transmission fiber, is doped with small amounts of dopants so that the inside layer has a slightly higher index of refraction than the tube itself. The preform tube is then heated so that it collapses to form a solid cylindrical preform rod, which is subsequently drawn so as to form a fiber. The central portion originating from the inside silica layer is the fiber core. The surrounding portions originating from the silica tube form the cladding and other layers. Such a silica fiber can be made as a single-mode fiber nearly transparent for radiation in parts of the 1.3 to 1.5 .mu.m wavelength band. Hence, such silica fibers have been widely implemented in telecommunications systems.
Residual absorption, however, does remain in silica fibers so that long telecommunication fibers, for example, greater than tens or hundreds of kilometers, require amplification of the original optical signal levels on the fibers. Until recently, a practical optical amplifier did not exist that could be combined with an optical fiber and therefrom the optical signal was regenerated, requiring conversion to an electrical signal.
However, a practical optical amplifier has been recently developed. The core of a silica fiber is doped with rare-earth Er.sup.3+ ions. If a short length of such a fiber carries both the modulated optical data signal at .about.1.55 .mu.m and an unmodulated optical pump signal at .about.0.8, 0.98, or 1.48 .mu.m, then the data signal is optically amplified. Such an erbium amplifier can easily be made into a fiber laser. Townsend et al. disclose a method of fabricating an erbium-doped fiber in "Solution-doping technique for fabrication of rare-earth-doped optical fibers," Electronics Letters, volume 23, 1987, pp. 329-331. They first deposit the core as an unsintered porous soot. Thereafter, an aqueous solution containing an erbium salt soaks into the soot, the aqueous solution being hydrolyzed. However, hydrolysis in an optical fiber layer introduces residual hydroxyl ions, which cause substantial absorption. More recently, the Er-doped glass has been modified by various combinations of the glass-formers Ge, P, and Al. Saifi et al. have disclosed co-doping with Ca, Al, Ge, and Er in "Er.sup.3+ -doped GeO.sub. 2 --CaO--Al.sub.2 O.sub.3 Silica Core Fiber Amplifier Pumped at 813 nm,"Technical Digest, OFC '91, 1991, p. 198. They soak a sooty inside CVD-deposited core layer with an ethanol solution in which are dissolved ErCl.sub.3, Al(NO.sub.3).sub.3, and Ca(NO.sub.3).sub.2 so as to form a calcium aluminum silicate core doped with Er and Ge. Such a glassy core improves small-signal gain when pumped at 800 nm. Even the best erbium-doped fiber amplifiers suffer disadvantages. Their gain spectrum is relatively narrow and uneven and cannot be moved to other parts of the infrared spectrum. Hence, other optically active ions are being investigated.
Doping silica cores with Er and other unusual elements presents a problem. The usual CVD fabrication techniques require that the dopants be available as gases or at least vaporizable liquids. Such sources are not readily available for such elements as Mg, Ba, Ca, Zr, and Pb.
Furthermore, optical fibers are needed that have a large numerical aperture, which is obtained by a large difference in the refractive index between the core and cladding. However, incorporating large amounts of dopants in the core by the gaseous-phase processes of CVD or flame hydrolysis is generally difficult.